One factor that pollsters must deal with is a dynamic electorate. While these changes can be significant at the national level, they may be even more pronounced within particular states. Indeed, we have heard a great deal about the large numbers of citizens that Democrats have registered over the past few years. But how big a difference has this really made? To get a sense of this, I gathered party registration data for seven swing states. The chart below compares the percentage of citizens registering as Democrats and Republicans in 2008 to the same figures in November, 2004 (obtained from the Secretary of State websites).
First, a disclaimer. Party registration is not necessarily comparable across states. For example, New Hampshire's open primary law allows independents to vote in either party's primary while Pennsylvania's closed primary rule means that you must register with a party to vote in that party's primary. Thus, the incentive to register with a party is greater in Pennsylvania than it is in New Hampshire (which is why Pennsylvania's figures are higher than in New Hampshire).
Nevertheless, the change (or stability) within each state provides important information and, in most cases, this information appears to favor Democrats. In percentage terms, Republican registration has declined in six of the seven swing states since 2004 (New Hampshire is the exception) while Democratic registration has increased in four of the seven states. Not every state has seen significant partisan changes. The partisan balance in Florida, New Mexico, and North Carolina is quite similar to what it was four years ago. But in four other states, the changes are more significant.
In two states that went for Kerry in 2004 (New Hampshire and Pennsylvania), Democrats have improved their standing among registered voters. In Pennsylvania, a state that the McCain campaign is still targeting, Democrats held a 7% advantage in party registration in 2004 compared to a 12% edge now. Democrats have also made big gains in two states that went for Bush in 2004--Colorado and Nevada. In Colorado, Democrats have cut the Republican registration advantage in half since 2004, from 6% to 3%. But Nevada provides the most stunning example of partisan change. In 2004, Republicans held a 1% edge in party registration; however, just four years later, Democrats now hold a 6% advantage.
These registration figures point to the difficulty that McCain faces in trying to pick up states like Pennsylvania and New Hampshire. Those states went for Kerry four years ago and have only become more Democratic since. The party registration statistics also indicate why Colorado and Nevada have become such good targets for the Obama campaign. In both states, Democrats have made dramatic gains.
What does all this mean for polling? We don't know for sure, but these dynamics do suggest some questions we should be aware of. For example, will these newly registered voters be included in pollsters' likely voter screens? Will those who are newly registered actually turn out on election day? Are all survey firms accounting for these trends when applying partisan (and demographic) weights to their samples? And--the key question--what will the changing partisan landscape in these states mean on election day?
Unfortunately, states like Michigan and Virginia do not have partisan registration so a similar analysis is not possible in those swing states.
UPDATE: A reader pointed out that in my original post, I mistakenly stated that North Carolina has a closed primary, which is not true. In fact, North Carolina does allow unaffiliated voters to vote in party primaries as long as the parties themselves agree. Both parties have agreed to do this in recent election cycles, so North Carolina has had semi-open primaries in recent years. The states that I've looked at here that have closed primaries include Florida, Nevada, New Mexico, and Pennsylvania.